1. Field of the Disclosure
The present disclosure relates to a method and a sensor for monitoring chemical species in a gas flow, Aspects of the disclosure relate to methods and devices for monitoring changes in odour; methods and devices for quality control of products; and methods and devices for filtering chemical species.
2. Description of Related Art
Numerous methods exist for monitoring chemical species in a gas flow. Such methods may be used for many different purposes; examples of such purposes include monitoring odours to detect, for example, contamination in food products, or in monitoring changes in air content to detect gas or other chemical leaks.
One of the more versatile methods used is ion mobility spectrometry (IMS), in which differences in the mobilities of ions in an electric field are used to distinguish one species from another. Typically a sample of gas is ionized, and passed through a flow channel which is subject to an electric field. Ions will experience different mobilities within the electric field depending on such factors as their charge, mass, and size. The time taken for ion species to pass through the field and arrive at a detector is calculated, and the resulting spectrum gives an indication of the ion species present in the sample. Generally the detector consists of one or more electrodes at the end of a flow channel or in the walls of the flow channel, with ions being detected once they have transited the flow channel and contacted a detector electrode. The location of the ion along the channel when it is detected can also be used to indicate the time of flight of the ion. The flow channel must therefore be sufficiently long to allow sufficient time of flight for separation of ions.
An alternative to this detection is field asymmetric ion mobility spectrometry (FAIMS). In this, an alternating (RF) asymmetric electric field is established across the flow channel; ions within the channel will be oscillated between the walls of the channel, eventually either passing through the channel or contacting the walls. The RF parameters under which ions will pass through the channel are a function of the differential ion mobility of the ion, so a FAIMS spectrum can provide information regarding the ion species present. This method has the advantage that a long flow channel is not necessary, thanks to the oscillation of ions; however, complex control electronics are needed to take account of this.
Both of these methods are fundamentally similar in that they rely on differences in ion mobility to distinguish ion species (that is, IMS makes use of differences in mobility over time, while FAIMS makes use of differences in differential mobility over frequency or other RF field parameter). The present disclosure, by contrast, aims to provide a simpler, less complex method for detecting or monitoring ion species. This is achieved, in part, through the determination simply of ion mobility, rather than differences in ion mobility; this allows the use of a DC electric field, and a relatively short flow channel while still retaining sufficient discriminatory power for certain desired applications.